1. Field of the Invention
Description
The invention concerns an electrohydraulic monitoring device for a dual effect (bi-directional) hydraulic cylinder.
The invention concerns an electrohydraulic monitoring device for a dual effect hydraulic cylinder, wherein the cylinder is divided into two pressure spaces by a cylinder piston, wherein the piston of the hydraulic cylinder and the cylinder housing are configurable into two alternative operating configurations for different functions by the alternative application and relief of pressure to the pressure spaces, and whereinxe2x80x94for examplexe2x80x94for safety reasons, the maintenance of a defined operating pressure is necessary, and having the further generic features set forth below. The monitoring device of this type is known from DE-PS 23 10 193.
2. Description of the Related Art
The above-mentioned known monitoring device is realized with an integrated construction, with a pressure reduction valve for adjusting the operating pressure configured in such a manner that functional elements of the pressure reduction valve are also used as functional elements of the monitoring device. The pressure reduction valve is received in a central segment of a borehole extending centrally through a longitudinally extending block-shaped housing. The pressure reduction valve includes a basically cylindrical pot shaped housing part sealed laminarly against the housing borehole, within which a valve piston is sealed laminarly and moveable by pressure within the housing part and supported via a radial flange against the free ring end surface of the cylinder jacket of the pot shaped housing part. A compression helical spring acts on the side of this piston opposite the valve housing, while for its part the pressure reduction valve opposing end is supported against a piston provided slideably pressure-tight in the housing borehole, of which the position within the housing borehole is variable in defined manner for adjustment of a defined pretension of this valve spring by means of a spindle drive means, of which a spindle nut provides the closure of the central borehole on the end of the housing block opposite to the valve. On the side of the pressure reduction valve opposite to the valve spring the central borehole is closed off by a cylindrical pot shaped housing closure part seated in the borehole in the base area fixed to the housing for closing the central borehole, wherein the ring end face of the tubular shaped jacket of the housing closure part facing the movable housing of the pressure reduction valve provides a housing fixed abutment for the valve housing, which demarks at the same time the zero setting or normal position of the valve housing as well as the end position, in which the valve housing of the pressure reduction valve is biased or pushed by the effect of the its valve spring. In this zero position of the valve housing a pushrod or tappet shaped projection of an xe2x80x9cinnerxe2x80x9d sensor piston projectsxe2x80x94extending through a central borehole of the base part of the housing closure part and sealed off thereagainst by means of a housing fixed ring sealxe2x80x94and seated pressure tight yet slideable within the inside of a tubular shaped segment of the housing closure part, and extending so far axially out of the housing closure part, that an electrical switch provides an electrical output signal characteristic of the position of the housing of the pressure reduction valve. This sensor piston is supported on the valve housing via a short axial rod-shaped projection on the valve housing, supported on the valve housing side. Between the flange shaped sensor piston setting off or delimiting the two rod shaped projections from each other and the base of the housing closure part, the pushrod shaped piston projection provided for switch operation is surrounded by a ring piston, which is sealed radially inwardly by a further ring seal against the pushrod shaped projection of the flange shaped piston and radially outwardly sealed by means of a sealing flange against the inner cylindrical jacket surface of the housing closure part, wherein this sealing flange on the one hand forms the axialxe2x80x94moveablexe2x80x94border or limit of a sensor chamber, which is bordered housing fixed by the base of the housing closure part, and at the other hand is bordered or limited by the axial border of a second sensor chamber, bordered by a flange shaped sensor piston supported directly on the valve housing and axially moveable. The housing internal space extending between the base of the valve housing and the sensor piston supported thereupon and the receiving space for the valve spring are in communication with each other via housing fixed channels and are thus under the same pressure as the central return flow circuit of the hydraulic system returning to the supply reservoir, which may be lower than that of the operating pressure to be coupled into the monitored hydraulic cylinder of the pressure reduction valve, however may be subjected to significant oscillations or pressure fluctuations of several bar, for example 5 bar. This type of pressure fluctuation could be suppressed by a special leak oil line, which however would be associated with additional expense and complexity. The sensor chambers separated pressure tight from each other by the ring piston are each individually connected to one of the pressure or operating spaces of the double acting hydraulic cylinder, which, depending upon operating mode of the hydraulic cylinder, are alternatively acted upon by the exit or output pressure of the pressure reduction valve, respectively, depending upon the switch position of the operating mode selection valve. The ring surfaces, upon which the sensor pistons are acted upon by the operating pressure existing in the hydraulic cylinder, are so dimensioned, that, as soon as in one of the sensor chambers an operating pressure of a defined high proportion of, for example, 80% of the setting of the valve spring as predetermined by presetting of the valve spring, is reached, which causes the flange shaped sensor position supported on the valve housing to slide relative to the valve housing, whereby the valve housing is axially deflected out of its base position or starting position relative to the housing closure part and with significant increase of the pretension of the valve spring, executes a deflection stroke a to a second position marked or defined by an inner abutment ring of the housing, which corresponds to a control position of the valve piston, in which the pressure supplied or coupled in to the cylinder is slightly higher than that pressure, which corresponds to the valve spring pretension, which was preset during the adjustment operation of the pressure reduction valve.
Independent there of which operating mode the main cylinder is being operated in, that is, in which of its operating or pressure spaces the operating pressure needs to be built up, this coupling in into one of its two sensor chambers always leads thereto, that the valve housing reaches its end position deflected by the stroke a relative from its xe2x80x9cpressurelessxe2x80x9d starting position. There in there is, depending upon in which sensor space the monitored pressure is coupled in, either the valve housing closest sensor piston alone is forced towards the valve housing and the other against the base of the housing closure part, or both sensor pistons are collectively pushed towards the valve housing. In both cases the rod shaped pushrod projection of the sensor piston on the valve side lifts off from the operating element of the switch, whereupon this, after the valve housing has carried out approximately a/2 of the deflection stroke, returns to its starting position, in which the switch starting signal indicates, that the operating pressure in the main cylinder is achieved, that is, the hydraulic device is operating as normal. If the actuating or operating pressure is not achieved for reason of some malfunction or defect, then this signal does not occur, and a signal combination remains established, which for example prevents that a machine is brought into operation, or if the signal first is established and then is lost, it is caused, that the machine is caused to switch off.
The known monitoring devices, on the as a result of their design and function as described above, are limited by at least the following functional disadvantages:
The ring seal, by means of which the pushrod shaped actuating element is sealed at its output side towards the switch is sealed against the housing closure part must remain sealed pressure tight against the relatively high pressure and thus must always be under a minimum pretension. This has the consequence that a relatively high friction occurs, which can be equivalent to a pressure of several bar. This seal thus has an influence of reducing the sensitivity of the monitoring. The friction resistance of this seal must be overcome, when both sensor pistons are to be displaced or moved collectively. This applies when the pressure need only be monitored in the respective main cylinder pressure chamber which is coupled into the base-side sensor pressure chamber of the monitoring device.
If in contrast the pressure must be monitored in that pressure space of the main cylinder, which is in communication with or coupled to the respective sensor chamber of the monitoring device which is bordered in the axial direction by the two sensor pistons, then the sensor piston on the valve housing side must be moved against both the friction of the outer ring seal as well as against that which unfolds against or is deployed against the ring seal of the ring shaped sensor piston, that is, the moveable piston must be deflected or moved against approximately twice the amount of the friction, with a consequence, that the sensitivity of the monitoring device, defined as the relationship Pintended/Pactual of the for adjusting the valve spring pretension preset intended operating pressure Pintended to the pressure Pactual, which is the minimal requirement in order to overcome or break the static or cohesive friction of the ring seal of the sensor piston, is reduced to approximately one half of the respective value, which applies for the first mentioned case. This is not of consequence so long as the monitored operating pressures are relatively high and the frictional resistant equivalent pressures are relatively small in comparison. If the monitored pressure for its part is however relatively low, then the possibility of malfunction or false positions is increased.
In addition to this, the known monitoring device during actuation, that is, as soon as the deflection stroke of the valve housing starts, tends to oscillate, since an axial displacement of the valve housing from a resting position of the valve piston, which corresponds to an equilibrium condition, to a sudden increase of the flow cross section of the xe2x80x9cvalve internalxe2x80x9d flow path of the flow cross section from the high pressure (P)-supply connection of the pressure valve to its user (A)-connection as well at the same time leads to a reduction of the flow cross section of the xe2x80x9cvalve internalxe2x80x9d flow path connecting the in the regulating operation the user (A)-connection with the return path (T)-connection of the valve, whereby a drastic unsetting of the equilibrium results, whereupon the thereby resulting oscillation can only xe2x80x9csettlesxe2x80x9d or decay after the valve finds a new control position.
It is thus the task of the invention to improve a monitoring device of the above described type in such a manner, that without interfering with the simplicity of construction, a significant increase in sensitivity of the monitoring device results which enhances sensitivity for both of the sensor pistons.
This task is inventively solved by the characterizing features of the present invention.
The sensitivity of the inventive monitoring device is essentially determined solely by the friction resistance, against which the sensor pistons and the actuating elements must be slid, which however can be maintained very small, since the relevant seals therefore need merely be capable of sealing against a relatively low pressure, so that significant pretension of this seal is not necessary.
The sensitivity of the monitoring device is practically the same for both operating modes or types of the hydraulic cylinder, since both sensor ring pistons can be sealed in laminar seals against, on the one hand, the slide guide tube or pipe in which they are received and, on the other hand, the central bore of the housing block, in which they are received, which besides the dampened moving friction in the actuated condition causes practically no friction.
The monitoring device is so designed that it can be used in combination with a conventional pressure reduction valve, since this, aside from requiring a small constructive change of the spring receiving space for preventing rotation of the adjusting jacket, in which the valve spring with its piston end is supported against the spring plate, requires no constructive changes, in contrast to the known devices, in which the valve housing must be made moveable, so that can be used as adjusting jacket for the monitoring device.
The design according to an embodiment of the present invention, with the narrow needle shaped actuating element extending through housing borehole as a step borehole, with sealing stage open towards a hollow space in which the ring flange of the adjusting jacket is received, in communication with the tank of the pressure supply assembly via the return flow circuit, of which the diameter is larger than the diameter of guide stage of this bore which is in communication with the oil-free switch receiving space, has the advantage, that, as described below, two lip seals can be received xe2x80x9csequentiallyxe2x80x9d by this sealing stage which for the already mentioned safety reasons preferably, particularly good sealing of the mentioned spaces relative to each other, at the same time double safety of the sealing, provide, which is further increased thereby, that between the two lip seals, preferably in the area of that one, which faces the oil filled space, a radial relief channel is provided, so that, in case the oil space side pressure seal is damaged, this event is recognizable by a trickling out of oil, before the danger occurs, that oil can accumulate in the switch receiving space.
The type of the rotational coupling according to a preferred embodiment of the present invention with the threaded spindle provided for adjusting the spring pretension with a rotatable however axially non-displaceable adjusting shaft is particularly suitable for a manual adjustment of the spring tension as well as for an electric motor adjusting thereof.
By the design of the housing jacket of the slide guide jacket of the threaded spindle provided according to a preferred embodiment of the present invention, there is achieved as a result, a pressure equalizing or balancing arrangement of the adjusting jacket within the through going borehole of the housing block of the monitoring device, such that pressure surges, which could occur in a common or communal return circuit of the hydraulic total system, and therewith also in oil spaces of the monitoring device in communication with this circuit, cannot have an effect on the position of the adjusting jacket (control casing) of the monitoring device.
The design of the surface relationship FS/FR of the control surface FS of the piston of the pressure reduction valve to the ring surface FR of the sensor positions of the monitoring device according to a preferred embodiment of the present invention results in a wide range of variation of the operating pressures predetermined by adjustment of the spring pretension to a reliable response of the monitoring device.
For this, under the context of the precise predetermination of the operating pressure to be monitored, in particular under the condition of relatively low operating pressures, the type of supporting of the threaded spindle according to a preferred embodiment of the present invention via a roller bearing against a valve spring axially supporting plate spring, of which a preferred design is discussed below.
This low friction design of the adjusting means provided for adjustment of the spring pretension is particularly of advantage, when for this an electric motor adjusting means is provided for adjustment, for which suitable design and arrangement possibilities are set forth below.
The preferred design of the monitoring device according to an alternative preferred embodiment makes possible in simple manner the recognition of malfunctions such as, for example, a blockage of the adjusting or control cylinder or also an increase of leakage loss in the operational circuit and, as a result, the implementation of supplemental safety functions.
It is particularly advantageous, when, as described below, a flow regulation valve is provided between the pressure reduction valve for setting the operating pressure and the pressure monitored hydraulic drive element, by means of which a defined value of a hydraulic oil flow streaming into a pressure monitoring drive cylinder is adjustable and therewith a defined basis for a comparison of a measured time parameter with a expected value associated therewith can be obtained.